1. Field of the Invention
The present invention relates to dynamometers for simulating the inertia and road load forces encountered by motor vehicles under anticipated driving conditions and, more particularly, to dynamometers of the type which utilize passive power absorption units, i.e., those which do not include motors coupled to the rotating parts.
2. Description of the Prior Art
Vehicle dynamometers are primarily used for two purposes: as measuring devices for determining the torgue and/or horsepower output of the vehicle, and as simulation devices for simulating the forces to which a vehicle is normally subjected during actual operation of the vehicle. The present invention is concerned principally with the latter application. The forces simulated include inertial forces, which are a function of the vehicle's weight and which must be overcome for the vehicle to accelerate or decelerate, and "road load" forces, which are those which must be overcome to maintain vehicle speed, and include such factors as break-away torgue, rolling friction and windage.
Dynamometers typically include a roll (or a pair of rolls) for engaging the driven wheel (e.g., motorcycle) or wheels (e.g., automobile) of the vehicle being tested. The roll or rolls are supported by a shaft or shafts journaled in bearings mounted on a frame.
Typically, a power absorber, such as a friction brake, an eddy current brake or hydrokinetic brake is coupled to the roll for absorbing power to the roll which, in turn, applies a retarding force to the surface of the vehicle wheel to simulate the road load forces. Inertial forces can also be simulated by such power absorbers during acceleration, but not during deceleration, since such absorbers do not supply power. Generally, where such power absorbers are used, the inertial forces are simulated by coupling the roll to one or more mechanical flywheels. The combined inertia of the flywheels and the roll (as well as the absorber) exert force on the vehicle wheels proportional to the acceleration (or deceleration) of the vehicle wheels. The combination of the flywheel (or flywheels) and the brake-type power absorber alone is referred to herein as a "passive" or "non-motoring" power absorption unit.
Electric motors have the capability of supplying as well as absorbing power and, for this reasons have been used to simulate both vehicle inertia and road load forces. Such a motor-type power absorbers supplier is referred to herein as an "active" power absorption unit. One or more flywheels may also be used in conjunction with such motors to minimize the size of the motor reguired.
When a vehicle is driven onto the dynamometer, the vehicle wheels are settled down between the rolls. When a test is completed it can be difficult to drive the vehicle back off the dynamometer rolls, since the wheels do not have sufficient traction to climb the curvature of the rolls, making it difficult to get the wheels back off the dynamometer. To alleviate this problem, dynamometers have been provided with lifts, which may be situated between the rolls of a roll pair and can be raised to engage the vehicle wheels and lift them off the rolls Such lifts are typically provided with an associated brake which brakes the rolls simultaneously with lifting the vehicle, so that the rolls cannot rotate. This makes it easy to then drive the vehicle off the dynamometer.
Dynamometers have to be periodically tested and calibrated to make sure that they operate properly. One of the calibration techniques commonly utilized is referred to as a "coast down test." For this test, the rolls are accelerated to a predetermined speed, and then the accelerating force is removed and the rolls are allowed to "coast down" from a first speed to a lower second speed while measuring the time required for this coast down. This is a simple matter for dynamometers with active power absorption units, since the motor can simply be used to accelerate the rolls and is then switched to a power absorbing mode during the coast down. However, in dynamometers with only passive power absorption units, there is no mechanism for accelerating the rolls to the predetermined speed.
In the past, calibration of such dynamometers has been attempted by driving the vehicle onto the dynamometer rolls, using the vehicle engine and wheels to accelerate the rolls to the desired speed, then using a floor jack to lift the vehicle and its wheels out of engagement with the rolls while the vehicle wheels are still spinning. (The lift which is part of the dynamometer cannot be used for this purpose, since it would make contact with the spinning vehicle wheels.) While this technique works, the procedure is considered to be quite dangerous Furthermore, this technique requires the use of at least two operators, one inside the vehicle to accelerate the engine and another to operate the floor jack.
Alternatively, it might be possible to utilize a separate electric motor along with the dynamometer to accelerate the rolls to speed, but the typical service station in which dynamometers are used do not have the requisite type of electric power available Thus, typically they do not have available three-phase power or 220-volt power. Thus, only a small motor, such as an 2-horsepower motor may be used. But such a small motor requires a long period of time (e.g., five or six minutes) to accelerate the rolls to the requisite speed and, even then, is only capable of accelerating them to between 30 and 35 mph. Furthermore, the motor must be stored somewhere and it adds an additional expense, because it must be utilized with a particular type of drive, such as a frequency inverter or flux vector inverter, which is fairly expensive.